1. Technical Field of the Invention
This invention generally relates to optical sampling, and more particularly to polarization-independent optical sampling with extended wavelength range.
2. Description of Related Art
Optoelectronics technology and its applications are expanding with the result that integrated optics technology can be used with considerable advantages in communications. Optical modulators, switches, multiplexers are commonly employed by fabricating them both on single substrates of both dielectrics and semiconductors. For measuring the waveforms of optical pulses used in high bit rate optical communications, it is common practice and desirable to use optical sampling with high sensitivity and high time resolution. Optical sampling systems often use a probe pulse signal and optical mixing with a user signal to achieve what is known as sum frequency generation (SFG) which is very useful for obtaining representations of sampled user signals. Some SFG methods of optical sampling might result in undesirably high background noise. Higher pump powers for optical sampling systems are desirable, but result in a greater need to eliminate background noise, because background noise increases with the square of the probe intensity. Also high powered probe signals are relatively more expensive.
It is to be noted that sum frequency generation (SFG) processes may use a nonlinear crystal such as for example, a periodically poled lithium niobate (PPLN) crystal. The use of a PPLN crystal in optical sampling systems is taught for example in the publication “Highly Sensitive and Time-Resolving Optical Sampling System Using thin PPLN Crystal” by S. Nogiwa, et al., Electron Lett, Vol 36, IEE 2000, which is incorporated herein by reference. PPLN crystals as opposed to other nonlinear crystals, e.g., KTP (potassium titanyl phosphate), have a large sum frequency generation efficiency under quasi-phase matching conditions. By a judicious selection of the PPLN crystal thickness, a reduction of the time resolution of the system to less than 1 ps and an increase of the wavelength sensitivity band width can be achieved. Experience has shown that the crystal length also partly influences performance characteristics such as acceptance and efficiency, both of which are inversely proportional to the crystal length.
Optical probe pulse sources are commercially available with a variety of features and applications with wavelength capabilities of 1550-1650 nm and some with 1100-1650 nm. Other wavelength capabilities for commercially available optical probe pulse sources are also known. Because of the availability of optical amplifiers in the 1550 nm wavelength band, there are several technologies available for obtaining short optical sampling pulses near 1550 nm. Examples of such include gain-switched semiconductor lasers and Erbium-Doped Ring Lasers.
High-speed sampling of optical signals facilitates reliable oscilloscope measurements of sampled signals. It has been found that a narrow sampling aperture enables achieving higher bandwidths than with regular electrical sampling techniques. Commercial optical sampling short-pulse sources near the 1550 nm range may indeed be obtained for this purpose. However, in most instances the input signals to be sampled also fall in the same 1550 nm wavelength band, which makes it difficult to distinguish the input signals from the probe or sampling pulse signals, compounding the background noise problem and, complicating the measurement system design.
In the optical signal sampling technology, it is to be noted that invariably the sampling signal or the probe signal is chosen to be much stronger than the user input signal, so as to obtain efficient conversion. For example, the user input signal could be of a magnitude of 1 milliwatt, whereas the probe or sample pulse may be 1 kilowatt peak.
The usual method of performing nonlinear conversion, as known, is by using a crystal that is designed to have a high conversion efficiency. Conversion efficiency of a crystal is influenced not only by the crystal length as discussed supra, but also by how precisely the conversion crystal axis is aligned with the polarization of the sampling pulse and user input signal. The sampling pulse or the probe signal is typically linearly polarized and generally does not pose any alignment issues. The user input signal on the other hand consists of two polarization components, one of which is not aligned with the crystal, resulting in a low output signal because of the polarization dependence.
Experimental attempts have been made to address the above described polarization dependence problem by using a manual polarization controller in front of the user input. The use of a manual polarization controller, however, is not practical in a commercial instrument or in a non-experimental situation.
In certain polarization deversity receivers on the other hand, the user input signal is split two polarized components, and the outputs are detected separately. The gain in each component route can be adjusted to compensate for polarization dependent loss (PDL).
One method of optical sampling without splitting either the sampling pulse or the user input signal wherein wider spectral acceptance is obtained using a shorter crystal and a three wavelength conversion scheme is described in co-pending U.S. application Ser. No. 09/885,154, which was filed on Jun. 20, 2001, and is now issued as U.S. Pat. No. 6,785,471, entitled “Optical Sampling Using Intermediate Second harmonic Frequency Generation”, which is incorporated herein by reference. It is noted in this context that just as stated in the co-pending U.S. Application, commercially available probe pulse sources usually are in the 1550-1560 nm range whereas user input signals in optical communications are in the 1550 nm range. In the co-pending U.S. Application however, efficiency of conversion of the optical input signal for sampling is somewhat sacrificed.
There is a need for a polarization-independent method and apparatus for optical sampling of a user input signal using a probe signal without sacrificing conversion efficiency and without a high degree of alignment problems. The need is especially felt when a user input signal comes from an optical fiber wherein the polarization state of the input signal is not known.